Human genetic diseases having a diverse range of consequences are caused by a newly discovered type of mutation. Genetic diseases such as Fragile X Syndrome, the major cause of mental retardation in the US, Huntington disease, Kennedy disease, and myotonic dystrophy, are all caused by an unusual expansion of a nucleotide triplet in the gene responsible for the disease. In the gene of normal individuals short tracts of a repeated triplet are present. A mutation, sometimes referred to as a "pre- mutation", then occurs that increases the tract length but is asymptomatic. Subsequently, a second mutation occurs that leads to much longer tract length and is associated with the disease state. A long term objective of the research is to understand how these expansions took place with the eventual goal of being able to influence the process to decrease the likelihood of such an expansion in individuals who are presymptomatic carriers of the so-called pre-mutation. The specific aims of the proposal are to develop several systems in the model eukaryotic organism, Baker's yeast (Saccharomyces cerevisiae) to understand several aspects of triplet repeat diseases. The first two aims of the proposal are to determine whether such expansions occur in Saccharomyces as our preliminary observations suggest. The third aim is to determine the consequences of such an expansion when it has been artificially introduced into the gene encoding the yeast transcription factor Adr1p. In particular, we will determine whether the mutation is dominant or recessive, and study its effect on the structure and function of the Adr1 protein. The fourth aim is to determine whether such an expansion is genetically stable when it has been introduced into the ADR1 gene. The final aim is to develop a selection scheme that will allow us to detect and study the properties of triplet repeat amplifications if they occur in yeast. By studying the frequency of a triplet repeat expansion in various mutant strains of yeast we hope to determine whether they are caused by errors during DNA replication or repair, and if so, to determine the enzyme that is responsible for the expansion.